AMSTERDAM (Reuters) - The Dutch public prosecutor is suing Royal Dutch Shell for not notifying authorities swiftly when one of its Dutch plants released a cloud of ethylene oxide into the atmosphere in March 2004.

Oil major Shell said the emission of the substance, which is highly flammable and can lead to explosions, took place by accident during the start up of the Moerdijk plant after a major turnaround.

A Shell spokesman said the firm initially thought the emission was less than 100 kilograms of the substance -- the threshold for informing authorities - but an investigation later showed between 1,000 and 2,000 kilograms was released.

"We are not happy about the incident itself, it shouldn't have happened," said a Shell spokesman on Thursday.

"We didn't know that it was more than 100 kilograms and as soon as we were able to calculate the exact amount then we notified the authorities immediately," he said.

He said a trial was likely to take place in a couple of months.

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The public prosecutor was not immediately available for comment.

Ethylene oxide is an intermediate product used in the production of other chemicals. Its derivatives are used in the making of bottles, clothing, soap and paint. Shell's Moerdijk plant is in an industrial area south of Rotterdam.